1. Field of the Invention
The present invention relates to a radar device for emitting a wave into space, receiving the wave reflected by an object present in the space, and performing signal processing on the received wave to measure the object.
2. Description of the Related Art
Generally, a radar emits an electromagnetic wave into space and receives the electromagnetic wave reflected by a target present in the space to know the presence/absence of the target, specifically, to detect the target. When the target moves relative to the radar, measurement of a frequency shift caused by a Doppler effect, specifically, measurement of a Doppler frequency also allows measurement of a relative velocity of the target, specifically, a Doppler velocity.
For the measurement of the Doppler frequency, an I/Q detection system for obtaining two orthogonal signal components as received signals is generally used. According to this detection system, each of a received wave and a local wave is divided into two to prepare two combinations of the received wave and the local wave. For each combination, the received wave and the local wave are mixed by using a mixer to obtain received signals in two channels. The two received signal channels are referred to as In-phase channel (I-channel) and Quadrature-phase channel (Q-channel), respectively.
For obtaining the Q-channel received signal of the received signals in two channels, a phase of any of the received wave and the local wave is rotated by 90 degrees. As a result, orthogonal components between the I-channel and the Q-channel are obtained. By performing a Fourier transform on a complex received signal obtained by regarding the I-channel as a real part and the Q-channel as an imaginary part, an amplitude of a frequency corresponding to a target Doppler frequency becomes larger. As a result, the target Doppler frequency can be obtained (for example, see “3. Radar and Its Environment” by R. J. Doviak and D. S. Zrnic, in Doppler Radar and Weather Observations, Second Ed., p. 30-53, Academic Press, Inc., 1993.).
When the received signal is obtained only for one channel, specifically, only for the I-channel, the received signal is a real signal. In this case, the Fourier transform of the received signal provides an amplitude distribution symmetrical about a frequency of 0. Therefore, even if the target Doppler frequency is positive, the amplitude becomes larger at two points, one of which is in a positive frequency and the other is in a negative frequency (the amplitude has two peaks in the frequency), after the Fourier transform. On the contrary, even if the target Doppler frequency is negative, the amplitude similarly becomes larger at two points, one of which is in the positive frequency and the other is in the negative frequency. Specifically, even if an absolute value of the Doppler frequency is obtained, a sign of the absolute value cannot be determined. Therefore, the sign of the Doppler frequency remains ambiguous. The ambiguous sign of the Doppler frequency means impossibility in determining whether the target is approaching or receding.
Similar ambiguity in the sign of the frequency also appears in a radar using a digital beam forming (DBF) system corresponding to a technique of synthesizing received beams through signal processing. In the DBF system, received signals obtained from a plurality of received elements are subjected to the Fourier transform in an element direction to obtain a signal distribution in an angular direction. Specifically, the DBF system is a technique of synthesizing the received beams through the signal processing (for example, see M. I. Skolnik, Introduction to Radar Systems, Third Ed., pp. 610-614, McGraw-Hill, 2001).
In such a DBF-system radar, when the received signal is obtained only for one channel, specifically, only a real received signal is obtained, an amplitude pattern of a received beam obtained by performing the Fourier transform on the received signal is symmetrical on a positive angle side and a negative angle side with a front direction defined as 0 degree. Specifically, it is uncertain whether an incoming angle of the received wave is positive or negative.
As described above, in the Doppler radar, when only a real signal (only for the I-channel) can be obtained as the received signal, the sign of the Doppler frequency cannot be obtained. Furthermore, when a DBF-system antenna is used, information indicating whether a target angle (defining the front direction as 0 degree) is positive or negative cannot be obtained. However, if a radar device is composed of the I-channel alone, the number of components advantageously becomes less than in the case where the radar device is composed of two channels, i.e., the I-channel and the Q-channel. Therefore, the radar device can be reduced in size as well as in cost.